1. Field of the Invention
The present invention relates to photolithographic projection and in particular to an exposure system, a test mask for monitoring polarization, and a method for monitoring the polarization.
2. Description of the Related Art
When a semiconductor device is manufactured with a lithographic process, it is important to optimize lithographic conditions, such as the polarization direction of light, dose, and the focus point for each of a plurality of masks. Resolution limit “R”, and depth-of-focus (DOF) of an optical system in an exposure system are given by following equations (1) and (2):R=k1λ/NA  (1)DOF=k2λ/NA2  (2)where “λ” is the wavelength of the light, “NA” is a numerical aperture of the optical system, and “k1” and “k2” are k-factors representing lithographic process other than the wavelength “λ” or the NA, such as resist properties. “k1” has a typical value of 0.61, and “k2” has a typical value of 0.5. As shown in the equation (1), a shorter wavelength “λ” and a larger NA permit smaller features to be printed. However, as shown in the equation (2), the wavelength “λ” and the NA are limited by the DOF requirement. Therefore, it is crucial to control the polarization direction, the dose, and the focus point to improve image fidelity under a shallow DOF. In Japanese Patent Laid-Open Publication No. 2000-310850, a method for monitoring the dose is proposed. According to the method, a periodic pattern having a pitch smaller than the resolution limit “R” is projected. Then, a line width of a projected image of the periodic pattern is observed to monitor the dose.